The mixing of, for example, ozone-containing gas into a pulp suspension, during ozone bleaching, usually takes place at a relatively high pressure since the prospects for the bleaching reaction are appreciably improved as the pressure in the reaction sector increases. Therefore, in order to be able to carry out ozone bleaching effectively, both the pulp suspension and the ozone-containing gas must be pressurized.
The apparatus required for pressurizing ozone constitutes by far the largest part of the investment costs associated with ozone delignification of pulp suspensions. The costs of such apparatus increases progressively in relation to increasing pressurization. When generating the gas, large quantities of energy are supplied to the carrier gas, for example pure oxygen, in ozone generators, in association with which a relatively small quantity of ozone is nevertheless formed since the carrier gas can only contain limited quantities of ozone. The total gas flow, which, as a consequence, is very large, is then compressed in so-called liquid ring compressors which are expensive and susceptible to disturbances.
In this context, the problem with currently known devices for bleaching pulp suspensions with ozone is that the reaction pressure which is possible is limited by the capacity of the compressors. In an example taken from a currently existing device, the compressors operate at an excess pressure of 10 bar. The pressure in the mixer may then, in practice, not exceed 7-8 bar excess pressure if blockage of the compressors at the slightest disturbance is to be avoided.
Nowadays, the admixture usually takes place by the pulp suspension being brought into rotation using a rotor surrounded by a coaxial stator, with the gas being supplied in the periphery of the rotating pulp suspension where the counter pressure for the compressors is greatest.